Interconnections between semiconductor-based integrated circuits, commonly referred to as “chips” and package leads are performed by wire bonding, solder bumps or tape automated bonding (TAB). Among them, the wire bonding technique is most commonly used because of its low cost. However, when the size of the chip-package interconnection scales down, the performance and reliability of wirebonding may be affected since wirebonding requires the routing of all the input/output (I/O) connections to the edges of the chip.
Solder bumping is the use of reflowable solder balls to join contacts on the chip with corresponding contacts on the package. It provides a useful alternative to the traditional wirebonding technology. The solder bumps are normally deposited on the bond pads located on the top side of the substrate that carries the chip. Between the solder bump and the chip, however, there is typically an UBM (under bump metallization) structure.
The UBM structure serves as an electrical and mechanical interface between the bond pad and the solder bump. It provides needed adhesion and also serves as a diffusion barrier between the solder bump and the bond pad.
Most UBM structures include multiple layers of metal or metal alloys. Copper is a commonly used metal in the UBM structures. It increases bondability and wettability of the solder. It is known that tin in the solder bump reacts with copper in the UBM structure by heat generated either in the course of reflow or during the use of the chip to form an intermetallic compound. Since the formed intermetallic compound is brittle, the bonding strength between the solder bump and the bond pad would be greatly compromised if copper is in direct contact with the solder bump.
Nickel has been incorporated into UBM structures to protect the copper layer. The UBM structures including the nickel layer prevent copper from coming in direct contact with the solder bump.
There is still a need for an UBM structure that has a thin copper layer while at the same time has good wettability and bondability.